This invention relates generally to blood gas monitoring, and more particularly to fiber optic reflectance sensors.
Early designs of fiber optic pH and CO.sub.2 sensors were done by Peterson, as described in the article "FIBER OPTIC PH PROBE FOR PHYSIOLOGICAL USE", by Peterson et al, published in Analytical Chemistry, Vol. 52, pp. 864-869, 1980 and by Vurek, as disclosed in the article "A FIBER OPTIC PCO2 SENSOR", by Vurek et al, published in Annals of Biomedical Engineering, Vol. 11, pp. 499-510, 1983. The pH sensor, described by Peterson, employed two optical fibers, connected to a semipermeable membrane pouch filled with phenol red dye bonded to a polyacrylamide powder substrate and mixed with reflective glass microspheres. The Vurek sensor employs the same dye (phenol red) in a bicarbonate buffer solution encased by a gas permeable, ion impermeable membrane. As carbon dioxide passes through the membrane, the pH of the buffer solution is altered.
In both the Peterson and Vurek sensors, the light absorption properties of phenol red are utilized in order to produce a color change, which can be monitored optically. Phenol red is a weakly ionizing acid which disassociates into an acid form having an absorption peak at 430 nm and a base form having an absorption peak at 560 nm. The proportions of acid and base forms are determined by the pH of the solution. Therefore, the pH of the solution containing phenol red can be monitored using light from a green LED, preferably having a frequency band centered at about 560 nm. In both sensors, the green light is provided to the sensor capsule by means of one of the optical fibers, and reflected light is monitored using the second optical fiber.
Recently, single optical fiber sensors employing fluorescence based dye systems have been introduced, as described in "OPTICAL FLUORESCENCE AND ITS APPLICATION TO AN INTRAVASCULAR BLOOD GAS MONITORING SYSTEM", by Gehrich et al, published in IEEE Transactions on Biomedical Engineering, Vol. BME-33, No. 2, pp. 117-132, 1986. This article is incorporated herein by reference in its entirety.